Skip to main content

Full text of "Circuits"

See other formats

Bacteria Battery 

Make] Projects 

hhiiilH ho/ 1 !/ tuMaal/ chare r\icf*f\\tat* 

build, hack, tweak, share, discover,- 

Bacteria Battery 

Written By: Ashley Franks 



Drill (1) 

(optional) if drilling your own lids 

Multimeter (1) 

(optional) for testing power 

Pair of Rubber Gloves (1) 

Shovel or Trowel (1) 

Wire strippers (1) 

(optional) if not using pre- stripped wires 

from the kit 

MudWatt MFC Kit (1) 
Item #MKKT01 from Maker Shed $45. 
Includes all materials below, except 
mud. You can also build MFCs from 
scratch, minus the Hacker Board. 

Container (1) 

Grommet (2) 

sized to seal marine wire through holes 

drilled in container lid 

MuddWatt Hacker Board (1) 
includes voltage- boosting chip. 8- pin 
socket. 22[xF capacitor, resistors, and 
red LED that blinks when bacteria 
generate power. (NOTE: some of the 
photos show the MuddWatt Explorer 
board instead.) 


Almost any mud with organic 
compounds will work. Try freshwater 
ponds or streams, brackish swamps and 
inlets, or saltwater marshes. Try to find 

© Make Projects 

Page 1 of 6 

Bacteria Battery 

mud that bubbles when disturbed or 
smells like rotten eggs — both are good 
indicators of anaerobic bacteria (but 
don't worry, your MFC won't smell). You 
can also use soil (just dig down a foot) 
or compost from the warm middle of a 

Marine Wire (2 lengths) 
Using 2 different colors helps. 


Microorganisms often get a bad name because some of them cause disease. But many have 
useful abilities, from making beer, cheese, and wine to processing waste and cleaning up 
toxic chemicals. One type of bacteria, discovered in 1987 by Derek Lovley, can generate 
electricity. Here's how you can find bacteria like these in a local pond and put them to work. 

Most non-photosynthesizing bacteria, like all animals, get their energy from the cellular 
respiration process, which converts glucose and oxygen to water and carbon dioxide. 
Oxygen works as an oxidizer, which means it accepts electrons as it combines with other 
chemicals in reactions. But special bacteria underground have no oxygen to breathe. 
Instead, they produce energy for their growth by transferring electrons to clumps of rust and 
other surrounding metal oxides, in a process called dissimilatory metal reduction. We now 
know that these electric bacteria are found in mud virtually everywhere on Earth, as well as 
in soil and compost heaps. 

A microbial fuel cell (MFC) does the same thing as a battery: drive electrons from an anode 
to a cathode through chemical oxidation/reduction reactions. What makes MFCs different is 
that they run on organic substrate and bacteria. 

"Metal-breathing" (Geobacter) bacteria at the anode carry out the oxidation reaction, 
converting plant and animal debris in the mud into electricity and carbon dioxide. Electrons 
flow through wires to a cathode sitting in water above the mud, where they combine with 
oxygen to complete the circuit. The bacteria are highly efficient in this arrangement and can 
produce electricity continuously for many months or even years. 

© Make Projects Page 2 of 6 

Bacteria Battery 

Experimental MFC- powered buoys now operate in the Potomac River, using naturally 
occurring bacteria in the mud to measure and transmit meteorological data. 

These "Benthic Unattended Generators" (BUGs) have worked for several years with no 
decrease in power output (see http://nrl . navy . mi l/code6900/bug ) . Geobacter species 
possess other useful abilities, such as the ability to respire radioactive uranium and remove 
it from ground water. They have proven versatile and effective in cleaning up areas 
contaminated with uranium or organic pollutants. 

In addition to their scientific interest, MFCs are a useful educational tool: a popular science 
project that encompasses microbiology, chemistry, electronics, and other disciplines. That's 
why Keego Technologies developed the MudWatt, a low-cost microbial fuel cell kit. They 
also support online discussion forums for MFC makers. 

With the MudWatt, students of all ages are learning about MFCs and making scientifically 
relevant discoveries. For example, a 6th-grade student in Santa Cruz uncovered (literally) a 
river sediment that produces twice as much power as typical topsoil. 

© Make Projects Page 3 of 6 

Bacteria Battery 

Step 1 — Make your own MFC. 

• Mix your mud (or soil or compost) to saturation with water and put a V2" layer into the 
bottom of your container. You can experiment by adding extra ingredients; see for ideas. 

• Cut the 2 pieces of graphite fiber cloth to fit the container. Be careful not to disperse the 
fibers in the air because they can cause short circuits in household electrical equipment. 

• Strip 4" from one end of each piece of marine wire, and weave each through a piece of 
graphite cloth. Strip the other ends V2" and thread each through a grommet. Drill the lid of 
the container to fit the grommets. 

Step 2 

• Lay one graphite cloth on top of the mud with its wire sticking up; this will be the anode. 
Cover it with V2" more mud and let it settle for a few minutes. 

© Make Projects 

Page 4 of 6 

Bacteria Battery 

Step 3 

• For the cathode, place the other graphite cloth on top of the mud, with its wire also pointing 
up. Avoid covering the cathode cloth with any mud, and gently pour a little water on top to 
moisten it. 

• If the cathode doesn't sink in water (it may have air bubbles in it), try putting a few small 
rocks on it to keep it from floating. 

• Close the container, sealing the grommets into the holes in the lid such that the wires stick 
out. It may help to gently twist the wires counterclockwise one turn before screwing on the 

Step 4 

• Attach the 2 wires to the MudWatt Hacker Board's 8-pin socket: anode to the (-) pin and 
cathode to the (+) pin. 

# Attach the Hacker Board to the lid of your container using the provided adhesive pad. 

© Make Projects 

Page 5 of 6 

Bacteria Battery 

Step 5 — Microbe Power 

• Within 3-10 days, the MFC will generally produce enough power to make the LED blink; 
the more power, the faster the blinking. Keep your MFC warm and moist inside to help 
the bacteria grow and produce the most power. 

• Try adding a very small amount of table salt to the water to increase conductivity if 
the voltages are too low. 

• The MudWatt Hacker Board boosts the bacteria's power to 2.4 volts, and its 8-pin 
socket lets you change a resistor value in the blinker circuit to optimize power to the 
LED. You can also power other devices that run on 3V or less. Download the 
instructions at Bo. . . and see for more ideas. 

• Power generated by an MFC is a product of its electrode surface area, so once you 
have your first one running, don't be afraid to go large. 

• By simply configuring 2 electrodes correctly in mud, soil, or compost, anyone anywhere 
can harness naturally occurring bacteria to provide continuous, non-polluting, carbon- 
neutral electricity. 

This project first appeared in MAKE Volume 30 , page 140. 

This document was last generated on 201 2-1 0-30 07:00:38 PM. 

© Make Projects 

Page 6 of 6